1. Field of the Invention
The present invention relates generally to a device and a method for controlling the position of the lateral edge of a continuous web of a recording medium in a printer or copier.
2. Description of the Related Art
In many printers or copiers, a continuous web such as a paper web is used as a recording medium, this web either being provided with a marginal perforation or not. If marginal perforations are provided, then the guidance of this web through the printer or copier is relatively easy since the lateral edge of the web is clearly defined by the engagement of pin-feed wheels in the marginal perforation. If the web has no marginal perforation, then a lateral displacement of the lateral edge can occur during the transport of the web through the copier or printer owing to inhomogeneities in the web, and the print image cannot be printed in a positionally accurate way.
In order to achieve a positionally accurate guidance, a web retracting device is provided in the entrance region of the printer or copier, the web retracting device exerting a restraining force on the web. As viewed in a transport direction of the web during the normal printing operation, an actuator in the form of a rotary frame is provided downstream thereof, which adjusts the position of the lateral edge of the web by rotation. Following the actuator, a web drive is provided which advances the web in the transport direction against the restraining force of the retracting device. Subsequently, the web is supplied to at least one transfer printing station in which toner images are printed on the web. Following the transfer printing station, a take-off device is arranged, which further conveys the web for further processing, e.g. for fixing the toner images.
During the transport of the web through the printer or copier, the lateral edge, as mentioned, can laterally drift away from the desired position. In order to position the print image in a positionally accurate way with respect to the lateral edge at the transfer printing station or at several transfer printing stations, the lateral edge of the web is controlled to a desired position with the aid of the rotary frame. For this purpose, a control device is used which processes signals of a sensor that senses the actual position of the lateral edge.
When guiding a paper web through a printer, lateral distortion of the paper web and a region-wise wave formation and/or single-edge sagging of the paper web can occur despite a stably running front edge of the web due to non-uniform mechanical web properties of the paper web or due to the basic setting of the various guide rollers not being exactly parallel. Such waves can be pressed into folds in the web at the web deflection locations in the transport apparatus, such as at counter-pressure rollers, for example, as required for transport of the web. Further, a single edge of the web may sag, for example, disturbing the image printing in the region of a non-contact fixing station since the sagging web section can come into contact with mechanical parts and the toner images are smeared as a result or the sagging section is subjected to an energy load that is too high.
U.S. Pat. No. 5,021,673 discloses a device for guiding a paper web in which rolls are provided at both lateral edges for guiding the web, the rolls exerting different pressure forces on the web. In this way, a lateral displacement of the web can be corrected.
U.S. Pat. No. 5,323,944 discloses a device for controlling the lateral position of a web, in which the web is guided through a pressure roller and a counter-pressure roller. The pressure roller is pivotable, and the force exerted on the counter-pressure roller along the axis can be varied in order to displace the lateral edge of the web. The current position of the lateral edge of the web is acquired with the aid of optoelectronic sensors.
U.S. Pat. No. 6,104,907 discloses a device for guiding a paper web in a printer. The paper web is guided and clamped around rollers in order to avoid vibrations and speed variations, as well as to counteract lateral displacement of the web. For example, for avoiding lateral displacement of the web, use is made of a guide roller having pins that engage into corresponding holes in the web. Another alternative provides to vary the force that a roller exerts along its axis on the paper web. In another alternative, the paper web is guided between pairs of upper and lower rollers. These upper and lower rollers contact and clamp the web with an increased angle of contact and thus avoid a speed variation of the web.